And there's a reason for their heft—aside from the obvious fact that big is beautiful.

"Bigger females can produce a larger number of offspring," says Greg Pauly, curator of herpetology at the Natural History Museum of Los Angeles.

For instance, female map turtles, native to the central and eastern U.S., can be twice as long and ten times the mass of males—all the better for toting more eggs.

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A female (left) and male common map turtle bask in the sun.

Photograph by Robert Hamilton, Alamy

In orb weavers and crab spiders, "females may be a hundred times bigger than males," Jo-Anne Sewlal, an arachnologist at the University of the West Indies, St. Augustine, says by email. In fact golden orb weaver males are so small they are often mistaken for offspring.

For Some Males, Smaller Is Better

Smaller male spiders can easily navigate through downed trees, rocks, and other aspects of a complex habitat to get to females, which stay put in their webs.

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A male and female golden orb weaver hang out on a web in South Africa's Kruger National Park.

Photograph by Edwin Remsberg, Alamy

A 2010 study showed that smaller spiders were more likely to "bridge"—walk upside down under self-made silk bridges—allowing for quick dispersal and access to more mating opportunities. Larger females were less likely to bridge.

Going over tree roots and other complex elements of landscape is also easier for smaller male turtles in search of females, Pauly adds.

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A pair of humpback whales swim together. Females, which are longer than males, reach lengths of 60 feet.

Photograph by Paul Nicklen, National Geographic Creative

Marine Life and Mammals

Tiny male marine creatures may have a difficult time finding a mate in the open ocean—even if she is many times larger.

Blanket octopus females, which inhabit subtropical and tropical oceans, can reach nearly seven feet in length, compared with males that are about 0.9 inch long. They're also 40,000 times heavier than the male, whose energy is better spent locating a female than growing big.

Octopuses 101How many hearts does an octopus have? How do species like the mimic octopus camouflage themselves? Find out about these and other octopus facts.